Not quite accurate.
iOS version 12, current beta release for developers is numbered 12. Same release for public beta is number 10.
The two betas are usually the same, but are distributed differently. Developer beta is usually released first, although, on some releases, both are made available at the same time.
Not pretty much: absolutely the same software. Same build number.
Regarding the number: Apple released the first beta to developers only. So that was Developer Beta 1. With the 2nd Developer Beta, they also released the identical software to the public beta testers. So Developer Beta 2 was also Public Beta 1. When Developer Beta 7 was released, it was pulled within hours and the public equivalent was never released. 2 days later, Developer Beta 8 was released and only then did the public get another beta, this time number 6. That’s why the numbers are 2 apart.